All weekend the threat of anti-climax hung over Lleyton Hewitt's head. After the euphoria of becoming the youngest world No1 when he beat Patrick Rafter on Friday night, the 20-year-old South Australian was heading for two more matches fraught with danger.

Mentally elated but struggling physically, it would have been so easy for Hewitt to take his eye off the ball. How many batsman get out immediately after reaching a hundred? How many boxers fall for the sucker punch? But Hewitt is tougher than that.

After disposing of Juan Carlos Ferrero in the semi-final 6-4, 6-4 on Saturday, Hewitt completed an amazing week by winning the Tennis Masters Cup at the Sydney SuperDome with a 6-3, 6-3, 6-4 demolition of Sébastien Grosjean in the final yesterday. Even by the standards set in this sports-mad nation, that was quite an achievement.

Not since Michael Stich managed it eight years ago in Frankfurt had anyone emerged unscathed from the round-robin phase and gone on to win the title. Hewitt not only won all of his five matches but did so for the loss of just one set - the very first he played on Monday evening against none other than Grosjean. After that came a performance which amounted to something close to perfection.

Both Ferrero and Grosjean had been playing well. The Spaniard had played superbly to beat Goran Ivanisevic in the last of his round-robin matches but against Hewitt his potent groundstrokes withered in his hand. Pushed back behind his baseline, Ferrero lost the penetration that had enabled him to overcome the top seed Gustavo Kuerten earlier in the week. The fact that Hewitt had almost defaulted with a strained hamstring in his left thigh did not seem to affect the young Australian one bit.

Admitting that the leg bothered him less yesterday, Hewitt rendered Grosjean helpless as well. "He was hitting so deep, I had to try and be aggressive. And then I made far too many mistakes," said Grosjean, who racked up 47 unforced errors. "It was difficult."

It tends to get very difficult when faced with a fiercely competitive opponent, backed by a joyously patriotic crowd in a match that had the odd hundred thousand dollars riding on it.

But Hewitt wasn't thinking about the money. Afterwards, $700,000 (£500,000) richer, he said: "I don't know what I'll do with it. Haven't got a house, haven't got a car. I'm pretty basic. Don't do a lot, actually, apart from support the Adelaide Crows and play a bit of golf."

If you are 20 and have risen to the very pinnacle of a game as physically wearing and technically demanding as pro tennis, you should be forgiven for not having had time to collect stamps or study the finer points of Aboriginal art. Mark Philippoussis, who had a head start on Hewitt as a potential sporting icon in Australia, developed an expensive interest in Maseratis. But his knees went and, despite a serve and a body that dwarfed little Lley ton, he never came close to becoming No1 in the world.

It is his mental strength as much as his surprising physical endurance that has stunned everyone from his own father, a former Aussie Rules footballer, to John Newcombe, his former Davis Cup captain. "I never expected him to achieve so much so fast," said Newcombe.

And the focus he brings to his game was still in evidence as he took a sip of champagne from the Waterford crystal Masters Cup. After all, the Davis Cup final is only 10 days away and that will mean yet another battle with Grosjean.

"I'll take a couple of days off and then start practising on grass," he said. "I've got to get my head down and try and fin ish off the year with a Davis Cup win. I love playing Davis Cup. It's the only opportunity you get to really show your colours in the locker room, to just hang out with the boys, go to dinner, play golf together. That's why I love it so much and it's one of the main reasons why I play so well."